1. Field of the Invention
The present invention relates to a phase-change memory device. More particularly, the present invention relates to a silicon on insulator (SOI) lower electrode in a chalcogenide memory cell. In particular, the present invention relates to an isolation device for a phase-change memory material that is formed in monocrystalline silicon on a buried insulator.
2. Description of Related Art
Phase change memory devices use phase change materials, i.e., materials that can be electrically switched between a generally amorphous and a generally crystalline state, for electronic memory application. One type of memory element originally developed by Energy Conversion Devices, Inc. of Troy, Mich. utilizes a phase change material that can be, in one application, electrically switched between a structural state of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. Typical materials suitable for such application include those utilizing various chalcogenide elements. These electrical memory devices typically do not use field effect transistor devices, but comprise, in the electrical context, a monolithic body of thin film chalcogenide material. As a result, very little chip real estate is required to store a bit of information, thereby providing for inherently high-density memory chips. The state change materials are also truly non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until reset as that value represents a physical state of the material (e.g., crystalline or amorphous). Thus, phase change memory materials represent a significant improvement in non-volatile memory.
One aspect of fabrication deals with the presence of parasitic effects in a bipolar isolation device. FIG. 1 is an array schematic for a high-density memory architecture that uses bipolar device isolation. At each cell location the data or bit line (BL, typically called the column line) connects to the phase-change structure, which is connected in series to a fixed resistor and then to a PNP bipolar emitter. The base of the emitter connects to a common row line (WL, typically called a word line).
FIG. 1 illustrates a vertical bipolar device that results in a degree of parasitic vertical capacitance. This capacitance results from the indigenous effect of isolation that is required in the P-N diodes. Some of the disadvantages include the fact that collector current goes to the substrate in the form of a collector or ground and must be collected at the array periphery. Since it is desirable to use these arrays with complementary metal oxide semiconductor (CMOS) technology such as N-channel and P-channel transistors in the die periphery, areas that consume P+ diffusion and N+ and Nxe2x88x92 (N well) guardbar structures must be provided around the array to reduce the possibility of latch-up. These structures reduce array efficiency and increase chip size and cost. Also the number of bits programmed in parallel may be limited due to the possibility of latch-up. Additionally, the bipolar gain, also known as the beta, of the parasitic vertical bipolar device will vary in conventional CMOS processing over a relatively wide range. This affects the amount of emitter current that goes to the row line instead of the substrate. Hence product design is more difficult and must be more conservative in order to harden the device against worst-case scenarios.
FIG. 2 illustrates an array schematic for a high-density memory architecture that uses both vertical 25 and horizontal 26 isolation. As a result, both vertical and horizontal parasitic bipolar effects cause a degree of reduced performance of the memory device. One disadvantage of a parasitic lateral bipolar device is that current passing through the emitter is collected by each collector. The bipolar gain, also know as the beta, of the parasitic lateral bipolar devices will vary in the conventional CMOS process over a relatively wide range. This affects the amount of emitter current that goes to the row line instead of the neighboring bits. Hence product design is more difficult and must be more conservative in order to harden the device against worst-case scenarios. Another challenge is that collector current then passes through neighboring unselected phase-change devices; a disturb current therefore passes through them. This phenomenon may result in affected retention time for data storage or the reliability of the memory state of these neighboring devices. Another challenge is that managing the lateral disturb requires technology trade-offs. The diodes must be separated by structures such as a deep trench. Alternatively or additionally, the diodes must be spaced apart more to minimize the parasitic lateral bipolar current. Another challenge is that there exists a parasitic capacitance between the base and the collector of each parasitic lateral bipolar device. This lateral parasitic capacitance slows the memory device because row line capacitance is increased.